Objective To evaluate the effect of intravenous dexamethasone on postoperative pain of microvascular decompression. Methods Totally 80 patients receiving elective microvascular decompression were randomized into two groups, with placebo or dexamethasone (10 mg, iv) given before the induction of anesthesia. The patients were generally anesthetized and the postoperative pain was managed with iv fentanyl using patient-controlled analgesia. The visual analog scale scores were recorded for pain at 2, 4, 6, 12 and 24 h after surgery and the side effects were also observed. Results The visual analog scale scores in the dexamethasone group were significantly lower than those in the placebo group at all time points(P<0.05). The total dose of fentanyl (0-24 h after surgery) and the frequency of analgesic use in the dexamethasone group were significantly lower than those in the placebo group(P<0.05). The incidence of nausea and vomiting in the dexamethasone group was lower than that in the placebo group(P<0.05).Conclusion Dexamethasone (iv) before anesthesia induction can intensify the analgesic effect of fentanyl, with less side effects.